Nickel titanium (NiTi) alloys have the properties of shape memory effect and superelasticity. The shape-memory phenomenon, first found nearly half a century ago, is that a material can exhibit one shape at a cold temperature and another shape after being heated to a higher temperature. The material is in its original shape at the higher temperature. When being cooled to a lower temperature, the material retains its original shape but changes the structure to martensite, where the material can be easily deformed into different shapes at the lower temperature. Upon heating, the material changes back to austenite, where the deformation is recovered and the shape is restored (one-way shape memory). Alloys can also have two memories (two-way shape memory) that exhibit a reversible effect, with heat causing the change in shape which can be reversed by cooling. The phase that is stable at the lower temperature is called Martensite (B19′); the phase stable at the higher temperature is called Austenite (B2).
The shape memory effect (SME) results from thermoelastic martensitic transition. Martensite is produced when austenite crystals in the parent matrix are cooled below the martensitic phase transition starting temperature (Ms). No macro shape change occurs at this stage, because of the formation of martensite twin in a self-accommodation structure. Twin boundaries can move and disappear when the martensite gains increased stress at a temperature below the martensitic phase transition finishing temperature (Mf), leading to macro deformation. The deformed martensite can be restored to the original shape of the parent phase through reverse transformation (from martensite to austenite) when being heated to a temperature above the austenitic phase transition starting temperature (As). Sometimes, martensitic reorientation can occur if the martensitic phase in the matrix is under increased stress. This phenomenon greatly contributes to the shape memory effect.
Super-elasticity (SE) or pseudo-elasticity (PE) occurs when a shape memory alloy shows a good performance at a temperature above the austenitic phase transition finishing temperature Af and is deformed at a temperature above Ms. For example, the best workable temperature range for PE is 10° C. to 15° C. above the Af. This effect is caused by the stress-induced martensite (SIM) formed at a temperature above Ms. As martensite is formed with stress applied thereto, the martensite reverts immediately to the undeformed austenite when the stress is removed. This process produces a “rubber-like” behavior in these alloys. This material will show two plateaus on the stress-strain curve in a tensile or compression testing, one in the upper (loading) section and the other in the lower (unloading) section, which are the regions of superelasticity.